ASIF Disk Images in macOS Tahoe
Disk images have been valuable tools marred by poor performance. In the wrong circumstances, an encrypted sparse image (UDSP) stored on the blazingly fast internal SSD of an Apple silicon Mac may write files no faster than 100 MB/s, typical for a cheap hard drive. One of the important new features introduced in macOS 26 Tahoe is a new disk image format that can achieve near-native speeds: ASIF, documented here.
This has been detailed as a major improvement in lightweight virtualisation, where it promises to overcome the most significant performance limitation of VMs running on Apple silicon Macs. However, ASIF disk images are available for general use, and even work in macOS Sequoia. This article shows what they can do.
Documentation is minimal at this point, and the macOS tools for manipulating ASIF disk images are limited compared with those for other formats, but this looks really promising.
Oakley has a table comparing the performance with other types of disk images when stored on internal SSDs, but the comparisons are not really apples-to-apples because they were made using different Macs. I did some quick benchmarks using the same Mac and SSD and got much slower results than he did in absolute terms, I think because I was using an external SSD connected via USB. However, in relative terms I found that .sparsebundle was about 50% faster at writing than .sparseimage and that .asif was about 1,000% faster. Read speeds were similar (and fast) among the formats. Benchmarking is tricky, especially with SSDs, so I don’t make any specific claims about what these numbers mean, but they are at least encouraging.
I’ve added preliminary support for ASIF disk images to the new public beta version of DropDMG.
Previously:
- macOS Tahoe 26 Announced
- DropDMG 3.6.9
- How Disk Images and VMs Are More Efficient
- Disk Images in Sequoia
